1. Field of the Invention
The present invention relates to a device for anchoring a rigid structure kept submerged in the subsurface by floats and anchored to the sea bottom by tension legs that are useful for supporting a plurality of arch-shaped support and guide elements referred to as troughs in a bottom-to-surface connection installation between a common floating support and the sea bottom.
More particularly, the present invention relates to an installation of multiple flexible bottom-to-surface connections between well heads, pieces of equipment, or the ends of undersea pipes resting on the sea bottom, and a floating support on the surface, the installation comprising a multiplicity of flexible lines, in particular flexible pipes, having their bottom ends connected to the ends of a plurality of undersea pipes resting on the sea bottom or directly to well heads or to pieces of equipment resting on the sea bottom.
In the present description, the term “flexible line” is used to mean pipes or cables capable of accepting large amounts of deformation without that giving rise to significant return forces, such as the flexible pipes defined below, and also cables or pipes for transferring power or information such as electric cables, control cables, or hydraulic fluid transfer pipes powering hydraulic equipment such as actuators, or pipes containing optical fibers; a flexible line may also be a control umbilical made up of one or more hydraulic pipes and/or electric cables for transmitting power and/or information.
The technical sector of the invention is more particularly the field of fabricating and installing bottom-to-surface connections for extracting oil, gas, or other soluble or meltable material or mineral material in suspension from under the sea, via a submerged well head, and up to a floating support, in order to develop production fields located off-shore at sea. The main and immediate application of the invention lies in the field of oil production.
2. Description of Related Art
In general, the floating support has anchor means enabling it to remain in position in spite of the effects of currents, wind, and swell. It also generally includes means for storing and processing oil together with off-loading means for discharging oil to off-loading tankers, which tankers call at regular intervals to take away the production. The common term for such supports is floating production storage and off-loading supports, and they are referred to throughout the description below by the initials FPSO.
However it is also possible for the support to be a semi-submersible floating platform installed temporarily at sea for a few years, e.g. while waiting for an FPSO type floating support to be built and installed permanently.
Bottom-to-surface connections with an undersea pipe resting on the sea bottom are known, that are of the hybrid tower type and that comprise:                a vertical riser having its bottom end anchored to the sea bottom via a flexible hinge, that is connected to a said pipe resting on the sea bottom, and that has its top end tensioned by a float submerged in the subsurface, the top end being connected to the float; and        a connection pipe, in general a flexible connection pipe, between the top end of said riser and a floating support on the surface, and, where appropriate, said flexible connection pipe under the effect of its own weight taking up the shape of a hanging catenary curve, i.e. going down well below the float before rising again up to the floating support.        
Bottom-to-surface connections are also known that are made by continuously raising strong rigid pipes up to the subsurface, such pipes being made of thick-walled tubular elements of steel that are welded or screwed together, and that take up a catenary configuration with curvature that varies continuously throughout the suspended length, which pipes are commonly referred to as steel catenary risers (SCR) or else as “catenary type rigid pipes” or as “SCR type risers”. Such a catenary pipe may go up as far as the support floating on the surface, or only as far as a float submerged in the subsurface that serves to tension its top end, which top end is then connected to a floating support by a hanging flexible connection pipe.
Bottom-to-surface connections are also known that enable a floating support to be connected to pipes or installations on the sea bottom that are constituted entirely by flexible pipes, in particular when the depth of water is not very great, e.g. lying in the range 300 meters (m) to 750 m, or even 1000 m, where the well heads or the pieces of undersea equipment are not very far from said floating support.
It should be recalled that the term “flexible pipe” is used herein to mean pipes, sometimes also known as “hoses”, that are well known to the person in the art and that are described in standards documents published by the American Petroleum Institute (API), more particularly under the references API 17J and API RP 17 B. Such hoses are particularly fabricated and marketed by the company TECHNIP-COFLEXIP France. Such flexible pipes generally comprise inner sealing layers of thermoplastic materials associated with layers that withstand pressure inside the pipe, generally made of steel or of composite materials and in the form of strips wound in touching spiral turns inside the thermoplastic pipe in order to withstand the internal bursting pressure, and associated with external reinforcement over the tubular thermoplastic layer and likewise in the form of strips that are spiral-wound with touching turns, but at a longer pitch, i.e. with a smaller angle of inclination for the helix, in particular lying in the range 15° to 55°.
Under such circumstances, each of said bottom-to-surface connections needs to be kept apart from its immediate neighbors in order to avoid any interference and any impacts, not only between floats, but also between flexible pipes and electric cables and other flexible lines such as electric cables or umbilicals transferring information signals and providing the connection with said floating support, when said flexible pipes are subjected to the effects of current, and when said floating support is itself subjected to swell, wind, and current.
In the development of certain fields, each of the well heads is connected individually to a said floating support and there are therefore very many bottom-to-surface connections, so it becomes impossible to install any more since the length of the side of the support is limited and as a result it can accept only a limited number of bottom-to-surface connections.
It is desired to install as many bottom-to-surface connections as possible from a given floating support in order to optimize the working of oil fields. That is why various systems have been proposed enabling a plurality of vertical risers to be associated with one another in order to reduce the occupancy of the working field and in order to be able to install a larger number of bottom-to-surface connections connected to a common floating support. Typically, it is necessary to be able to install up to 30 or even 40 bottom-to-surface connections from a common floating support.
Documents WO 02/66786, WO 02/103153, and WO 2011/061422 in the name of the Applicant describe hybrid towers with multiple flexible pipes and risers arranged in fans enabling a large number of connections to be associated with a common floating support in spite of the problem of the movements of said risers interfering with one another since they are all subjected to the same movement as their top tensioning floats under the effect of the movements of the floating support on the surface where it is subjected to swell, wind, and currents.
In those installations, proposals are made to arrange two flexible pipes that are superposed or arranged side by side between the floating support and the top ends of risers or SCRs, the two flexible pipes being guided in the subsurface by two respective troughs fastened in superposed or laterally offset manner to a float for tensioning a third riser that is located closer to the floating support than are the first two risers, each said trough thus defining two flexible pipe portions in the form of hanging double catenaries on either side of the trough. That configuration presents the advantage of making it possible to bring the flexible pipes to the top end of the riser that is relatively far from the floating support without the bottom points of said hanging double catenary pipe portions being too deep.
When a multiplicity of bottom-to-surface connections are used that are constituted exclusively by flexible pipes, it is also necessary to space the various connections apart from one another, at least for the following reasons.
Firstly, flexible pipes have fragile outer sheaths, and it is essential to prevent them from striking against one another.
Secondly, the flexible pipes are used by passing via arch-shaped guide elements referred to as “troughs”, each defining a rigid bearing surface of convex curved shape as described below, so as to define two flexible pipe portions, comprising a first flexible pipe portion in a hanging double catenary configuration between the floating support and said trough, and a second flexible pipe portion in a single catenary configuration between said trough and the point of tangential contact between said flexible pipe and the sea bottom.
Those arch-shaped guide elements referred to as troughs are well known to the person skilled in the art, they present:                a longitudinal section of curved shape in section in the axial vertical longitudinal plane of the trough, preferably a section of circular shape with its concave side facing towards the bottom of the sea, and a convex outside surface on which the pipe is placed; and        a cross-section in the vertical plane perpendicular to the vertical axial longitudinal plane of the trough presenting a shape with a curved bottom that is preferably circular with its concave side facing upwards and constituted by said top outside surface lying between longitudinal side walls serving to hold and guide the pipe in the longitudinal direction between said side walls.        
In known manner, the radius of curvature of the longitudinal curve with its concave side facing downwards is greater than the minimum radius of curvature of the pipe passing via said trough.
Such a trough serves to impart controlled curvature to the portion of flexible pipe that it supports so as to avoid excessive curvature which would irremediably damage said pipe.
The function of such troughs and the arrangement of the flexible pipes serves to create a hanging double category curve on the upstream side of the trough between the floating support and the trough so as to avoid or reduce as much as possible the stresses and movements of the flexible pipes at their point of contact with the sea floor which would destructure the sea floor by creating trenches and would weaken the pipe because of the pipe being flexed in alternation in the region of the point of contact, thereby requiring its structure to be reinforced and/or requiring the sea floor to be protected. The stresses and movements at the point of contact between the flexible pipe and the sea floor are indeed reduced as a result of the stresses and the movements of the pipe being damped by the first flexible pipe portion in the form of a hanging double catenary that is created by causing the pipe to pass over said trough, the first portion being more involved in absorbing horizontal movements of the floating support than is the second flexible pipe portion in the shape of a single catenary.
When suspended from its two ends, a said undersea flexible line takes up under its own weight the shape of a hanging double catenary, as is known to the person skilled in the art, i.e. it goes down in a catenary configuration to a low point where its tangent is horizontal (see below), after which it rises up to said floating support, which hanging catenary can accommodate large amounts of movement between its ends, which movements are absorbed by deforming the flexible pipe, in particular in the rising or descending portions on either side of the low point of said hanging catenary.
It should be recalled that the flexible pipe portion between an end from which it is suspended and the low portion of horizontal tangent, specifically in said second flexible pipe portion the point of contact with the sea bottom, adopts a symmetrical curve as formed by a hanging pipe portion of uniform weight subjected to gravity, which curve is known as a “catenary” and is a mathematical function of the hyperbolic cosine type:y=R0(cos h(x/R0)−1)R=R0·(Y/R0+1)2 where:                x represents the distance in the horizontal direction between the horizontal tangency point and a point M on the curve;        y represents the height to the point M (x and y are thus the abscissa and ordinate values of a point M on the curve relative to a rectangular frame of reference having its origin at said point of contact);        R0 represents the radius of curvature at said point of contact, i.e. the point with a horizontal tangent; and        R represents the radius of curvature at the point M(x,y).        
Thus, the curvature varies along the catenary from the top end where its radius of curvature has a maximum value Rmax to the point of contact with the sea floor where its radius of curvature has a minimum value Rmin (or R0 in the above formula). Under the effect of waves, wind, and current, the surface support moves laterally and vertically, thereby having the effect of raising or lowering the pipe of catenary shape where it touches the sea bottom.
For a bottom-to-surface connection in the form of a single catenary, the most critical portion of the catenary is situated in its portion close to the point of contact, and most of the forces in this bottom portion of the catenary are in fact generated by the movements of the floating support and by the excitations that are applied to the top portion of the catenary, which is subjected to current and to swell, with all of these excitations then propagating mechanically along the pipe to the bottom of the catenary.
The essential function of the first portion of the flexible pipe in the form of a hanging double catenary that is located upstream from the trough is thus more specifically to absorb, at least in part, the movements of the pipe and/or the movements of the floating support to which said flexible pipe is connected, by mechanically decoupling movement between respectively said floating support and said second flexible pipe portion in the form of a single catenary. However another function is also to reduce the traction forces exerted by said second flexible pipe portion on the undersea equipment and/or the end of the pipe resting on the sea bottom to which it is connected, as the case may be.
In the prior art, the intermediate support troughs for said flexible pipes are held in the subsurface at a certain depth by supporting floats from which each of the troughs is suspended. However those floats are subjected to large amounts of movement which means that sufficient distance must be provided between the various floats in order to ensure that they do not strike against one another.
Those constraints involve spreading out the working zone and limiting the number of flexible bottom-to-surface connections that can be connected to a common floating support, via its sides, in order to avoid interference between the various flexible connections and the various floats. That is why it is desired to provide an installation suitable for making it possible from a given floating support to use a plurality of flexible type bottom-to-surface connections, with reduced size and movement, and that is also as simple as possible to lay, being suitable for being fabricated at sea from a pipe laying ship.
WO 00/31372 and EP 0 251 488 describe pluralities of bottom-to-surface connections in which flexible pipes extend from a floating support to the bottom of the sea, passing via a rigid support supporting a plurality of troughs all arranged at the same height side by side with lateral offsets, said troughs being supported by a said support structure resting on the sea bottom or by a said support structure suspended from floats and connected by not more than two tension legs to a base anchored to the bottom of the sea. The top and bottom ends of the tension legs are connected to the support structure at the attachment points of the support structure and to the base at the respective attachment points thereof referred to as “top attachment points” and “bottom attachment points”.
In the prior art, it is sought to minimize the number of tension legs between the trough support structure and the base at the sea bottom, so as to be left with no more than two top attachment points in alignment at the trough support structure and therefore no greater number of tension legs aligned in a generally vertical common plane at their top ends so as to obtain a mechanical connection that is isostatic. If three tension legs are used, maintaining an isostatic connection requires the three top attachment points of said tension legs that are parallel to one another and substantially vertical to be in an arrangement that is triangular, preferably an equilateral triangle, in a plane that is not a substantially vertical plane. The more said triangle becomes flattened, i.e. the more its vertex tends towards an angle of 180°, the more it moves towards a configuration of three points in alignment and the more it moves away from an isostatic configuration.
Mechanically, a configuration is said to be “isostatic” when the distribution of forces in said tension legs is unique and therefore computable in known manner. However, for three tension legs having top attachment points that are in alignment, and also for more than three tension legs, the mechanical system ceases to be isostatic and becomes statically undetermined, i.e. the distribution of forces in each of the tension legs cannot be calculated in unique manner. In this non-isostatic example, as for a four legged stool, the assembly may possibly become unstable, with some of the tension legs possibly carrying a greater fraction of the load, while others are less loaded or even in some cases completely slack, i.e. they carry no load.
In the above-described prior art, rupture of a tension leg, leads either to the destruction of the installation when there is only a single tension leg, or to dangerous unbalance of the trough support structure when there are two tension legs or when there are three tension legs arranged in a triangle in a plane that is not substantially vertical. This generally leads to the trough support structure tilting to a large extent or completely, thereby running the risk of irremediably damaging the flexible pipes or electric cables it supports and thus leading to partial or total destruction of the bottom-to-surface installation. In addition, for crude oil production lines, such incidents risk causing major pollution.
Such ruptures are particularly to be feared in installations where the depth of water is not very great, i.e. a few tens or even hundreds of meters, since at those depths, swell and current act on the entire depth of water and are thus particularly troublesome for a trough support structure fitted with its troughs and buoyancy elements. In addition, swell, wind, and currents also destabilize the floating support, and the resulting movements are transmitted via the flexible pipes to the submerged support structure and therefore have an effect on the tension legs and on their top and bottom attachment points. Thus, when the depth is not very great, i.e. up to a depth of 300 m and when ocean and weather conditions are dangerous, the flexible pipes, the trough carrier structure, and its connections to the foundation on the sea bottom are particularly subjected to forces that are considerable, or even extreme, thereby creating wear and fatigue, mainly at the ends of the tension legs and at their attachment points. Such accidents have already happened in the recent past.
A known but unsatisfactory solution for making a multiple tension leg system less statically undetermined consists in designing a trough support structure that presents great flexibility, i.e. that is able to bend considerably, which thus enables all of the tension legs to contribute, but with certain limits. The main drawback of that configuration lies in the problems of fatigue and wear that are to be feared at the tension legs and their attachment points, then being transferred to the support structure and thus potentially leading, in the event of an incident, to even worse damage.